The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
The applicant has previously disclosed a system for measuring and mapping a surface relative to a reference surface in the publication WO 2007/000010. The system, amongst other things, provides the use of a scanner to gather scan cloud data, and then measure and map that scan cloud data relative to a reference surface, which in the specific embodiments described, is the internal surface of a rotating cylindrical mill. As disclosed therein, despite the scanner being placed as close to the centre of the mill as possible, the positioning will never be exact. Accordingly, the base reference data and the scan cloud data need to be aligned with the greatest degree of accuracy as is achievable.
In the comminution of minerals within the mining industry, the crushed ore is separated into pieces and may then be fed into rotating cylindrical mills. The rotation of a mill about its axis causes the ore pieces to tumble under gravity, thus grinding the ore into decreasingly smaller fractions. Some types of grinding mills are fitted with secondary grinding systems such as iron or steel balls (ball mills), steel rods (rod mills) or flint pebbles (pebble mills) which assist in the grinding process.
The accurate measurement of wear in a mill is particularly important, as the cost of incorrectly predicting which liners to replace is great. If the nature of the wear can be identified accurately, wear patterns may also be identified to optimise the operation of the mill.
Other measurements, such as the volume of the secondary grinding system, the size of the components thereof, and the size of screening systems within a discharge arrangement, are also difficult to measure.
Current means of measuring the ball charge volume are achieved using hand held tape measures, to measure the distance from the surface of the ball charge to some estimated longitudinal central axis; and to measure the diagonal distance across the surface from one corner to the other.
These measurements are then used to calculate a volume using simple geometry. A problem with this methodology is the fact that the surface is never perfectly flat and the base reference data is not known. For example, the liners covered by a ball charge are ill defined. Unless the reference plane, above which the surface of a ball charge sits, is known precisely, the calculation of ball charge volume will inevitably be inaccurate.
The volume of the ball charge is an important factor in achieving optimum grinding and production from a mill. A variation of only 4% from the optimum ball charge has been reported to reduce throughput by between 5% and 10%. Improving the accuracy of this measurement will improve the level of control and performance of the mill.
Furthermore, there is no known method to determine what proportion of the remaining objects in a mill are attributable to the materials being ground as opposed to the objects of the secondary grinding system. One method to measure the ratio of steel balls to ore is to empty the mill, separate the components, and weigh them. Another known method is to monitor the gross mass flows of ore, water and balls into the mill. However it is difficult to separate out these components from each other in exit streams, so the best result achieved is a rough estimation. The ratio of ball to ore is very important for the optimum performance of the mill. If there is too much ore then the balls take more time to crush it to the correct size and throughput is limited. If there are too many balls relative to the ore they risk impacting on bare liners and dramatically increasing wear rates.
In the case of measuring the size of the components or objects that make up a secondary grinding system, there is no optimal method to measure their individual size. In a ball grinding system, the only known way to measure the size of these balls is to physically remove them from the mill and, as previously mentioned, individually measure them. This is impractical in an operational mill, due to the significant downtime incurred.
Balls can be added to mills on a daily basis and the quantity and size are a critical performance parameter. Ball size is important because as balls wear and reduce in size their impact power is reduced and they are less effective in breaking down rocks. Typically when the diameter of a sphere has been reduced to its half original size the mass will be reduced by a factor of 8. At this point the steel ball is considered ineffective and simply occupies valuable space in the mill, limiting production. Smaller balls will also produce a finer product and the design of the mineral liberation system downstream of the mill is highly sensitive to product grind size. The matching of ball size to product sizing and the control of these parameters is often critical to the yield of the resulting mineral.
Finally, measuring the size of screening system employed in a mill is important in order to gauge the performance of the mill. Grates constituting the screening system hold back the grinding media. As grate holes wear they will allow larger grinding media through and reduce the effectiveness of the grinding process. The only known method of measuring these grates is to manually enter the mill and measure the grate holes individually. Typically there are several hundred grate holes in each mill. Given the repetitive nature of the tasks of manually measuring hundreds of grates, the process is also highly susceptible to human error.